1. Field of the Invention
The present invention relates to a signal processing apparatus and the method for performing contour correction adaptable to a frequency band of luminance of an image, and an image display apparatus using the signal processing apparatus.
2. Description of the Related Art
As one of image processing, there is known contour correcting processing for correcting to a clear image by enhancing a contour of an image with an indistinct contour or blurring a contour of an image with a harsh contour to some extent. As one kind of contour correction, for example, in order to improve sharpness of an image on a display of a television set, etc., contour enhancing processing for enhancing high frequency characteristics by extracting high frequency components of a video signal by filtering processing, giving a suitable gain and adding the same to the original video signal is general.
When contour enhancement is visually recognized, sharpness of an edge portion composing the contour in an image is determined in accordance with an amount of enhancement (a gain of a correction amount), on the other hand, it is necessary to prevent a portion around the contour from becoming unnatural due to excessive contour enhancement on the portion around the edge portion. On an object of preventing such unnaturalness, the present inventors have already proposed a video signal processing apparatus, wherein a suitable filter coefficient is calculated in accordance with a luminance difference of adjacent pixels, so that a bandwidth (for example, a half bandwidth) of a bandpass filter and a center frequency to be corrected can be changed, and a gain of a contour correction signal output from the filter in accordance with the luminance difference can be adjusted. The video signal processing apparatus is described in The Japanese Unexamined Patent Publication No. 2000-013642 (the patent article 1).
This technique is, as described in the patent article 1, to determine whether an image of a scene to be subjected to contour correction includes high frequency components or not based on whether a luminance difference of adjacent pixels exceeds, for example, a certain threshold or not in an image (input image) of a specific resolution to be subjected to processing by a color signal processing apparatus, such as a video camera, and to change filter characteristics and gains in accordance with the determined result.
In a field of a display apparatus in recent years, a display unit, such as a display panel, have become higher in resolution, and an input image adapted to the display resolution can be sufficiently clearly displayed, so that the case of not requiring contour correction has increased. On the other hand, there is a demand for contour correction adaptable to a variety of image signals output from various kinds of apparatuses and having different standards on resolution, etc. and different frequency bands.
A variety of signals, such as a standard-definition (SD) image, a high-definition (HD) image, a broadcast image wherein sharpness changes in accordance with a signal path, high quality image of a digital versatile disk (DVD), etc., and an image mixed with graphics from a Set Top Box (STB), etc. are input from a variety of input terminals to a processing block for performing image signal processing on sharpness, etc.
As a method of determining a signal type in the processing block, a method of determining by from which input terminal the signal is input, and a method of determining by a frequency of a horizontal and vertical synchronization signals, etc. are common. Although, a signal type can be determined by that, it is difficult to also learn the frequency band of the signal precisely. For example, in the case where the input terminal is a D4 terminal, images with different resolutions, such as an SD image and HD image, are input, so that the signal band cannot be determined based only on the input terminal. It is also the same in an RCA terminal. Also, when determining by a synchronization signal frequency of an input signal, a signal bandwidth cannot be determined only by a frequency of a synchronization signal because a signal bandwidth of a broadcast wave image passed through a variety of paths is different from that of a DVD image even if their frequencies of the synchronization signals are approximately same.
On the other hand, in a recent display apparatus on the basis that a variety of types and resolutions of images are input, a function of adapting an input image to a resolution of the display portion by enlargement scaling by performing interlace progressive conversion (IP conversion) is provided. In this case, as a result that data is newly created between pixel data of original input image with a low resolution by interpolation processing at the time of enlargement scaling to adapt the image to a display portion of a high resolution, a contour of the image is liable to blur. In this point, contour enhancement becomes necessary, and a recent display apparatus is provided with a signal processing apparatus (signal processing IC) having a function of contour enhancement in a processing block after IP conversion.
There is a case where even when an image input to the contour enhancement processing block is input, for example, from a D terminal and a signal frequency after IP conversion is equivalent to that of an HD image, the image at the time of being input and before the IP conversion is an SD image. Therefore, even when determining by combining determination by an input terminal and by synchronization signal frequency explained above, a frequency band of the signal cannot be correctly determined.
As explained above, when performing contour enhancement by a signal processing apparatus of the related art on the basis that a variety of signals are input, images having different frequency bands are input to the same input path of the contour enhancement processing block. In this case, contour enhancement is not necessary when both of the input image and display image are HD images or other high resolution images, but when the input image is an SD image and the display image is an HD image, sharpness of the image deteriorated by IP conversion, etc. has to be corrected by contour enhancement. However, in a current signal processing apparatus, a signal bandwidth cannot be determined due to the reason explained above, so that contour enhancement by a required amount adaptable to the signal bandwidth cannot be performed. Namely, in the case of setting a contour enhancement amount to a sufficient level for an image with a narrow frequency band so as to correct sharpness of the deteriorated image by IP conversion, when an image with a broad frequency band is input, the contour is excessively emphasized and the display image becomes harsh and poor. As a countermeasure thereof, it can be adjusted by a user, but the user has to understand relevance of a frequency band and contour enhancement, which is difficult and not realistic.
Next, an explanation will be made on the case of applying the technique described in the patent article 1 to contour enhancement of the signal processing apparatus on the basis that a variety of image signals are input.
A luminance difference of adjacent pixels is detected in the technique described in the patent article 1, but this is processing for determining whether a high frequency component of a certain value or higher is included or not as a reference for switching the filter characteristics and contour enhancement gains, but not for accurately detecting the signal bandwidth. Also in this case, a contour enhancement amount is switched in the case of including a high signal bandwidth to a certain degree, and in the case of a low signal bandwidth, but it is not possible to follow changes of a signal bandwidth of the input image, and sufficient accuracy cannot be obtained on setting a contour enhancement amount adaptable to the signal bandwidth.
Furthermore, there is a case where, for example, a contour enhancement amount is changed every time a scene of an image changes and a way of contour enhancement frequently changes on the display screen. In this case, it results in an even poorer display image.